Various methods for producing shape memory polymers are known. For instance, the shape of a polymer material can be changed reversibly and isothermally by controlling its pH, by chelation, or by using chemical energy generated by a redox reaction. The shape of the polymer material can be changed reversibly and isothermally by using photoreaction of the photosensitive groups; heat is also used for this purpose. Several kinds of polymers having a heat-sensitive shape memory property have been known. These polymers can be classified by their structure into polymer crosslinked substances having the proper melting point or glass transition temperature (above a room temperature) or cold-worked materials of polymer crosslinked substances having the proper melting point or glass transition temperature and a remarkably high molecular weight. In typical polymer materials within the temperature range below the glass transition temperature or the melting point, thermal motion of the molecular chains is restricted so that the polymer becomes hard. Once this polymer is heated to exceed the glass transition temperature or the melting point, it becomes a rubber-like substance. Such a temperature dependence is common among polymer materials. In view of practical use, there are problems in the temperature region of the glass transition temperature or melting point, and the level of deformation.
Almost all polymers, however, have shape memory properties if they have substantial crosslinking temperatures to the extent that strains are not relieved. A kind of polymer resin produced by any suitable production method is subjected to a crosslinking reaction in order to memorize its shape after molding. The mold is heated to a temperature above its glass transition temperature or melting point, and the resin is deformed and cooled below its glass transition temperature or melting point while keeping its deformed state so that the strains will be maintained. The thermal motion of the molecular chain is restricted and its strain is fixed at or under the class transition temperature or the melting point. When this deformed mold is again heated to be at least glass transition temperature or melting point where the molecular chains can do thermal motion, the strain is relaxed and the original shape is obtained. Such well-known shape memory polymers include crystalline polyolefin crosslinked substances (U.S. Pat. No. 3,086,242), crystalline trans-isoprene crosslinked substances (Japanese Patent Application No. 61-16956) and crystalline trans-polybutadiene crosslinked substances (U.S. Pat. No. 3,139,468). Among the polyolefins, crystalline polyethylene crosslinked substances are used for heat-contracting tubes. In these crystalline polymers, however, crystallization is not obstructed by crosslinking. As a result, special operations are required to provide shape memory properties, for instance, crosslinking is provided by low temperature vulcanization or irradiation on to the crystallized polymers. When the molecular weight of the polymers is remarkably high, the shape memory property can be found since the intertwining polymer chains substantially define crosslinking temperatures and thus, the strain is not relaxed even if the temperature is at or below the glass transition temperature. Well-known examples of such shape memory polymers include polynorbornene (JPA No. 59-53528), poly(vinylchloride), poly(methyl methacrylate), polycarbonate, and acrylonitrile-butadiene (AB) resin. These shape memory polymers are used for mechanical devices, heat-sensitive tubes and portions that should recover the original shapes after absorbing shock. Examples of articles using such materials are toys, deformed pipe jointing materials, laminate materials inside pipes, lining materials, clamp pins, medical instruments, teaching materials, artificial flowers and car bumpers, mechanical devices and heat-contracting tubes. Although separation membranes using such shape memory polymers have been disclosed (JPA 2-645), a reversible property of the shape memory polymers has not been applied for a separation membrane. Therefore, such membranes are not practically used.
For a separation membrane, optional bore size control, namely, designing a porous membrane having several separation properties is difficult when the conventional polymer materials and the membrane forming technique are used. In addition, the conventional separation membranes, especially porous membranes, have another problem: the permeability will deteriorate because of fouling. In order to solve such problems, back wash has been carried out. Back wash is a technique to apply pressure from the permeation side (the direction opposite to normal pressurizing direction) to remove fouling substances. In this back wash technique, however, the pressure for the treatment must not be set high in order to avoid membrane damage, and thus, a sufficient wash-recovery effect cannot be obtained.